1. Field of the Invention
The present invention generally relates to a digital demodulation technique for burst signals according to a digital modulation scheme in a wireless digital communications system, and in particular to a timing clock recovery circuit and method thereof.
2. Description of the Related Art
In digital demodulation for burst signals, it is necessary to perform clock synchronization using a preamble of each burst signal. An example of such a digital demodulator for burst signals has been described in Japanese Patent Unexamined Publication No. 5-110612.
The conventional digital demodulator is provided with a quasi-sync quadrature detector which detects the real part and imaginary part of a complex-modulation baseband signal from a received radio signal. Hereinafter, the real part and the imaginary part are called real signal S.sub.RE and imaginary signal S.sub.IM. respectively.
The real signal S.sub.RE and the imaginary signal S.sub.IM are respectively converted into digital form by analog-to-digital (A/D) converters depending on a sampling clock signal which is generated by a timing clock recovery circuit. The digital real signal S.sub.RE and the digital imaginary signal S.sub.IM are complex-multiplied by recovered carriers to produce I-channel demodulated signal and a Q-channel demodulated signal, respectively.
The timing clock recovery circuit is provided with a clock phase error detector and a voltage-controlled oscillator (VCO) for burst clock recovery. The clock phase error detector detects a clock phase error from the I-channel demodulated signal and the Q-channel demodulated signal. Based on the clock phase error, the sampling clock signal is generated and output to the A/D converters by the VCO for burst clock recovery, so that the clock phase error is compensated for.
In the case of mobile communications, as described before, since radio channel characteristics frequently vary, it is necessary to transmit the preamble signal for each burst transmission to compensate for variations in a radio propagation characteristic. Especially when the sync operation is started at the beginning of the burst signal, there may be a large clock phase error between the recovered timing clock signal and the clock component of the received burst signal. In this case, a long preamble causes rapid synchronization but reduced throughput of the communications system.